A lighting circuit of a discharge lamp of a metal halide lamp or the like for an illuminating light source for an automobile, can include a direct current power source circuit having a constitution of a DC-DC converter, a direct current-alternating current converting circuit, and a starting circuit. For example, a direct current input voltage of a battery can be converted into a desired voltage in the direct current power source circuit, and then converted into an alternating current output by the direct current-alternating current converting circuit at a post-stage. The alternating current output is superposed with a starting signal that is supplied to the discharge lamp (see, for example, Japanese Patent Document JP-A-7-142182).
To light the discharge lamp, a no load output voltage (hereinafter, referred to as ‘OCV’) is controlled before lighting the discharge lamp (i.e., when lamp is turned off) and a starting signal is applied to the discharge lamp. The lamp is shifted to a steady state lighting state while reducing a transient input power.
For example, a switching regulator using a transformer can be used for the direct current power source circuit. As the direct current-alternating current converting circuit, a full bridge-type arrangement having pairs of switching elements or the like can be used.
One result of the arrangement for carrying out two-stage conversion (i.e., direct current voltage conversion and direct current-alternating current conversion) is that the circuit scale is enlarged, which is not suitable for small-sized formation. To obtain a reduced size, an output stepped up by one stage of voltage conversion in the direct current-alternating current converting circuit to the discharge lamp can be provided.
For example, one arrangement includes a series resonating circuit using a capacitor and an inductance element for supplying power to a discharge lamp after stepping up a resonant voltage by a transformer. The series resonance of the capacitor and the inductance element provides a frequency characteristic that is substantially symmetrical and centered on a resonance frequency. An output voltage or power can be controlled by changing a drive frequency of a semiconductor switching element forming a direct current-alternating current converting circuit. At a frequency region higher than the resonance frequency (inductive region or delay phase region), the output voltage tends to be reduced by increasing the frequency. At a frequency region lower than the resonance frequency (capacitive region or advance phase region), the output voltage tends to be reduced by reducing the frequency.
According to OCV control in turning off the lamp (before turning on the lamp) after inputting the power source, at a frequency region higher than a series resonance frequency (which is described as ‘Foff’), an OCV value is increased by reducing the drive frequency of the semiconductor switching element. When the value reaches a target value, a starting high voltage pulse is generated and applied to the discharge lamp. When the discharge lamp is turned on, the discharge lamp shifts to a frequency region higher than the series resonance frequency (Fon>Foff) to start a control of power of the discharge lamp.
When a lighting frequency is increased by high frequency formation to reduce the size of the circuit apparatus for supplying a voltage in a shape of a rectangular wave or the like to light the discharge lamp, at a predetermined frequency or higher, resonance of vibration of a gas inside the discharge tube (sound wave) and a discharge arc pose a problem. A disturbance in a shape of the arc is initiated by a so-called ‘acoustic resonance’ phenomenon. The frequency of occurrence of the phenomenon is determined by a shape of the discharge tube and a pressure of the gas inside the discharge tube.
At a lighting frequency equal to or higher than 1 MHz, a frequency of occurrence of acoustic resonance is not continuous but discrete and, therefore, a stable arc is provided by setting the lighting frequency at a frequency which does not bring about acoustic resonance. Individual settings differ depending on the differences in the shape of the individual discharge tubes. A method of setting individually the lighting frequency for each discharge tube is not realistic in mass production formation.
It would be helpful to restrain acoustic resonance in lighting a discharge lamp at high frequency.